Photochromism means such characteristics of a material that the material develops a color under irradiation with excitation rays such as ultraviolet rays and returns to the initial uncolored state when allowing the material to stand or the material is irradiated with infrared or visible rays or heated after the ultraviolet rays irradiation. That is, photochromism means that the material is reversibly colored and discolored repeatedly.
Photochromic compounds have the above-mentioned characteristics, and the color density of the compounds in the coloration and the discoloration differs depending on the intensity of the irradiated excitation rays and the time of irradiation, so that they have been widely used for various materials such as window glasses in architectures and sunglasses, utilizing their photochromic characteristics. Further, photochromic materials using such photochromic compounds have been tried to apply to image-forming materials for copying, recording or the like.
The photochromic compounds used for the photochromic materials are broadly classified into organic photochromic compounds and inorganic photochromic compounds.
Most of the inorganic photochromic compounds are solid and in the form of grains. Further, the specific gravity of the inorganic photochromic compounds is larger than that of organic high-molecular compounds, so that it is difficult to dissolve or homogeneously disperse the inorganic photochromic compounds in the organic high-molecular compounds. On the other hand, the organic photochromic compounds can be homogeneously mixed with the organic high-molecular compounds, so that they can produce photochromic films having uniformity in the coloration and the discoloration, and also produce photochromic laminates in which a photochromic layer having uniformity in the coloration and the discoloration is laminated on a substrate.
However, some problems still resides in the organic photochromic compounds. For example, if the organic photochromic compound is irradiated with excitation rays for a long period of time, the compound is sometimes oxidized to suffer deterioration. Further, if the organic photochromic compound is oxidized, the compound loses the photochromic characteristics of the reversible coloration and discoloration.
Therefore, various methods have been proposed to improve the photochromic composition containing the organic photochromic compound. In concrete, for restraining the deterioration of the organic photochromic compound, there have been proposed a method of adding hydroxybenzophenone, hydroxybenzotriazole or derivatives thereof to the composition as described in U.S. Pat. No. 3,212,898, a method of adding a hindered amine compound thereto as described in U.S. Pat. No. 3,488,290, and a method of adding a nickel complex thereto as described in Japanese Patent Laid-open Publication No. 58(1983)-173181. Also proposed are a method of adding a thioether compound to the composition as described in Japanese Patent Laid-open Publication No. 58(1983)-113203 and a method of adding a hindered phenol compound or a phosphite compound thereto as described in U.S. Pat. No. 3,488,290.
However, ultraviolet absorbing agents such as hydroxybenzophenone and hydroxybenzotriazole absorb ultraviolet rays required for exciting the organic photochromic compound to develop a color, and thereby the light sensitivity of the photochromic composition lowers, or the deterioration of the organic photochromic compound caused by application of light is accelerated depending on the kind of the organic photochromic compound or the ultraviolet absorbing agent used.
A hindered amine light stabilizer (HALS) is known to have plural functions such as a function as a singlet oxygen quencher, a function as a radical-trapping agent and a function as a hydroperoxide decomposing agent. However, a photochromic composition containing the hindered amine light stabilizer and the organic photochromic compound is insufficient in the light-resistance, and hence the photochromic composition is required to be much more improved in the light-resistance in the case where the resulting photochromic material is expected to be used repeatedly.
A photochromic composition containing the nickel complex and the organic photochromic compound is also insufficient in the light-resistance, and additionally, the photochromic composition is colored owing to the nickel complex even when the light irradiation is not conducted.
Further, in the case where a hindered phenol compound, a phosphite compound or a thioether compound is used instead of the hindered amine light stabilizer or the nickel complex, the resulting photochromic composition is also insufficient in the light-resistance.